A systematic low-temperature neutron diffraction study of the RBa₂Cu₃O_x(R=yttrium and rare earths; x=6 and 7) compounds

Abstract

The crystal structures of RBa₂Cu₃O_x (R=Y and rare earths; x=6 and 7) ceramic materials were investigated at 10 K by neutron diffraction and consistently analysed concerning systematic trends. Other than for non-superconducting PrBa₂Cu₃O₇ the lattice parameters and most interionic distances exhibit the well known lanthanide contraction behaviour, i.e., a linear relationship with the ionic radii of trivalent rare-earth ions. The only exceptions are associated with the apex oxygen O(1) ions: the chain copper Cu(1)-O(1) distances are constant within error limits, and the plane copper Cu(2)-O(1) distances are increasing across the rare-earth series. The much stronger increase of the distance Cu(2)-O(1) in the RBa₂Cu₃O₆ series compared to the RBa ₂Cu₃O₇ series can be explained by the increase of T_c from 90 K for YbBa₂Cu₃O₇ to 96 K for NbBa₂Cu₃O₇. The smaller distance Cu(1)-O(1) for the RBa₂Cu₃O₆ series compared to the RBa₂Cu₃O₇ series may be related to the suggested double-well potential of the apical oxygen ion. For some interionic distances of PrBa₂Cu₃O₇ approximately parallel to the h direction (i.e., the chain direction) we determine by extrapolation a valence of +3.4 for the Pr ions. This indicates for PrBa₂Cu ₃O₇ a highly anisotropic 4f-CuO₂ valence band hybridization. An important structural property with respect to the superconductivity is the puckering of the CuO₂ planes: the superconductivity is lost when the puckering angle exceeds a critical value of about 167.3°.